1. Field of Invention
This invention is related to a single source solid precursor matrix for producing semiconductor nanocrystals and doped semiconductor nano crystals for displays and a process for preparation thereof. The doped semiconductor nanocrystals of II-VI compounds generated in situ within an inorganic layered precursor (LP) and particularly doped ZnS nanocrystals surface capped with ZnO micro-shell, both generated and remain embedded within the decomposed zinc-thiourea-sulfato-hydroxide precursor matrix. The composite material shows efficient photo- and electro-luminescence suitable for making multicolor displays and bio-markers.
2. Description of Related Art
The luminescent semiconductor nanocrystals, generally termed as nanophosphors, are employed as the materials for displays, bio-labels, and lighting applications [Bhargava R N and Gallagher D, “Optical properties of Mn doped nanocrystals of ZnS”, Phys, Rev. Lett. Vol. 72, 416 (1994)]. The specific advantages of nanophosphors include the possibility of simultaneous synthesis and doping of the nanocrystals through low-temperature (room-temperature) wet chemical methods with high luminescence quantum efficiency, short radiative decay-time, wide emission color tunability, etc.
Although, the process for making nanophosphors are still evolving, however, the most known synthesis processes include reverse micelle route, synthesis of organometallics and colliodal precipitation therefrom, sol-gel process, etc. In the reverse micelle route [(Counio G, Esnouf S, Gacoin T, Boilot J P, “CdS: Mn nanocrystals in transparent Xerogel Matrices; Synthesis and Luminescence Properties”, J. Phys. Chem. Vol. 100, 20021 (1996)], variety of nanoparticles including CdS, CdSe, ZnS, PbS, etc, are prepared in a size restricted water-pool of the water-in-oil ternary system. Though the method was widely used in the early stages for making intrinsic II-VI group nanoparticles, but it was found not suitable for making doped nanoparticles because of the unavoidable use of excess surfactant medium which hinder the doping process. The excess surfactant also causes difficulties in the separation of nanoparticles from the reaction medium for any technological applications. Yet another disadvantage of this process is that the surface capping agents used such as mercaptoacetic acid, mercaptoethanol, etc (thiol compounds, in general) are toxic in nature. The process also suffers from low-yield of the precipitate.
Preparation of ZnS:Mn, ZnSe:Mn and CdSe:Mn nanoparticles by the reaction involving organometallics are reported in a number of patents and publications [(U.S. Pat. No. 5,525,377 by Ghallagher et al, US patent application no. 2002/0011564 A1 by D. J. Norris and the reports from Bhargava et al Bhargava R N and Gallagher D, “Optical properties of Mn doped nanocrystals of ZnS”, Phys, Rev. Lett. Vol. 72, 416 (1994), and Mikulec F V, Kuno M, Bennani M, Hall D A, Griffin R G, Bawendi M G, “Organometallic synthesis and characterization of manganese-doped CdSe Nanocrystals”, J. Am. Chem. Soc., Vol. 122, 2532, (2000), Suyver J F, Wuister S F, Kelly J J and Meijerink A, “Luminescence of nanocrystalline ZnSe:Mn” Nano Lett. Vol. 1, 429 (2000)]. Many nanoparticle systems prepared through this method are already available in the market for its use as bio-labels and medical imaging phosphors. However, this method has only limited acceptability because of the use of costlier and often poisonous organometallic chemicals. Moreover, these chemicals are rarely available or have to be freshly prepared in-house through highly controlled and stringent chemical reactions (U.S. Pat. No. 5,525,377). This results in the high cost of the products (Approx. US$500 per 10 mL of the suspension: Ref: Ocean Optics, USA, Catalog-2005). Another disadvantage of this method is the use of toxic hydrogen sulfide (H2S) gas (in case of sulfide nanocrystals) as the source of S2-ions. A further disadvantage of this process is that the organometallic precursors of dopants such as Mn2+, (diethyl manganese) are highly unstable and either polymerize or precipitate as separate phase during the co-precipitation reactions. Yet another disadvantage of this method is that the known art does not provide a common reaction route for the incorporation of multiple dopant ions in the semiconductor lattice, which is necessarily required for multicolor-emission from the nanophosphors. The process is also time consuming as the dissolution of the surfactants or capping polymer (e.g. PMMA) in organic solvent (toluene) takes about 12-18 hours. Further, this process necessitates an additional step of post-synthesis UV curing of the nanoparticles.
Another known preparation of nanoparticles include sol-gel synthesis and colloidal precipitation in hydrocarbon or aqueous solvents under various environmental conditions. Artemyev M V, Gurinovich L I, Stupak A. P and Gaponenko V, “Luminescence of CdS nanoparticles doped with Mn” Phys. Stat. Sol (b) Vol. 224, 191 (2001), Ali Azam Khosravi, Kundu M, Jatwa L, Deshpande S K, Bhagawat U A, Murali Shastri, Kulkarni S. K, “Green luminescence from copper doped zinc sulphide quantum particles”, Appl. Phys. Lett. Vol. 67, 2702 (1995) and Wang M, Sun L, Fu X, Liao C and Yan C, “Synthesis and optical properties of ZnS:Cu (II) nanoparticles”, Solid State Commun. Vo. 115, 493 (2000) discuss the preparation of CdS:Mn2+ (in DMF and 1-mercaptopropyltrietoxysilane gel), ZnS:Mn2+ (inert atmosphere with mercaptoethanol capping), ZnS:Cu2+ (in glycine aqueous solution), respectively. Although these methods are adequate for the preparation of nanosized particles, however, they do not represent a convenient and generalized methodology for doping of different impurity ions having various valence states. The methods also suffer from the disadvantage of immediate surface oxidation of nanoparticles which invites the requirement of inert atmosphere for the preparation.
A major disadvantage for the nanoparticles systems prepared by all the aforesaid known processes is that they are highly dispersible in air and water and therefore cause potential environmental threats. Recent toxicological studies (David B. Warheit; ‘Nanoparticles: Health Impacts’ Materials Today, February 2004) show that the nanoparticles inhaled during the synthesis, processing and/or applications produce highly adverse inflammatory responses when compared to bigger particles of the same chemical composition. The inhaled nanoparticles deposit in the lung's wall and slowly escape the lung's defense (alveolar macrophage) surveillance system and transmigrate into the interstitial regions of the lung, causing long standing respiratory tract diseases including tumor. This problem is applicable to all kinds of nanoparticles and particularly more severe in the case of nanoparticles of highly toxic character, such as CdS, CdSe, PbS, etc. Recent studies of in vivo cytotoxicity of CdSe containing colloidal nanoparticles (both bare as well as core-shell) (Austin M Derfus, W. C. W Chan and Sangeeta N. Bhatia, “Probing the cytotoxicity of semiconductor quantum dots”, Nano Lett. Vol. 4, 11 (2004) confirms the acute toxicity of these nanocrystals due to the release of free Cd2+ and Se2+ by way of disintegration of nanoparticles within the biological cells causing carcinogenic mutations. These findings reveal the main limitation of presently available luminescent nanoparticles.
Many environmentalist groups have already called by implementing moratorium or ban on the research and development of toxic nanoparticles.
Thus, the existing methods and materials related to luminescent nanoparticles are attended with disadvantages and drawbacks described hereinabove.